1. Field of the Invention
This invention relates to pneumatic tires, and more particularly to a so-called high-performance pneumatic tire realizing improvements of wet performance and steering stability and reduction of noise without sacrificing other tire performances.
2. Description of Related Art
In order to simultaneously satisfy high wet performance and low noise, it is generally useful to adopt a so-called directional tread pattern defined by arranging in a tread portion circumferential grooves extending straight in a circumferential direction of the tire and a plurality of slant grooves extending obliquely in a direction of converging from each of ground contact ends of the tread toward a pattern center.
In order to further improve the wet performance, it is effective to render the slant groove into a steeply slant groove having a small inclination angle with respect to the circumferential direction of the tire. In this case, however, land portions defined by these steeply slant grooves have necessarily an acute portion, and hence the rigidity of the land portion lowers and steering stability is degraded.
In addition, it is effective to increase a negative ratio for improving the resistance to hydroplaning. However, the increase of the negative ratio undesirably increases the noise level.
In order to improve the steering stability, it is useful that the slant groove is a gently slant groove extending obliquely at a large inclination angle with respect to the circumferential direction of the tire, or the arranging pitch of the slant groove is made large to enlarge the land portion defined by these slant grooves. In this case, however, the gently slant groove causes the degradation of pattern noise and hence the sufficient drainage performance at the ground contact region of the tread, particularly central region thereof is not obtained.
In the conventional tire having the aforementioned tread pattern, it is difficult to simultaneously and balancedly establish the wet performance, steering stability and noise.